U.S. patent number 7,495,609 [Application Number 11/594,521] was granted by the patent office on 2009-02-24 for mobile gps aiding data solution.
This patent grant is currently assigned to eRIDE, INC.. Invention is credited to Arthur N. Woo, Guenter Zeisel.
United States Patent |
7,495,609 |
Woo , et al. |
February 24, 2009 |
Mobile GPS aiding data solution
Abstract
A mobile GPS-aiding system uses a GPS reference receiver to
collect GPS navigation messages, a GPS-aiding data network server
to distribute over the Internet all ephemeris and almanac data
gleaned from the navigation messages, a number of commercial
broadcast radio stations to publish such ephemeris and almanac data
on particular sub-carriers, a number of vehicles equipped to
receive the radio broadcasts and the sub-carriers and to retransmit
them locally, e.g., via Bluetooth. Portable GPS receivers, operated
near any of the vehicles, a receive Bluetooth transmissions with
the ephemeris and almanac data with the identity of the radio
broadcast station then being tuned. A breadcrumb database is used
to index the locations of the radio broadcast stations. Each mobile
GPS receiver contributes to such database after it computes a
location fix. If the location of the radio broadcast station is
already known to the database, then the location can be accessed
and used before finding a position solution.
Inventors: |
Woo; Arthur N. (Cupertino,
CA), Zeisel; Guenter (Ebersberg, DE) |
Assignee: |
eRIDE, INC. (San Francisco,
CA)
|
Family
ID: |
40364631 |
Appl.
No.: |
11/594,521 |
Filed: |
November 7, 2006 |
Current U.S.
Class: |
342/357.64 |
Current CPC
Class: |
G01S
19/05 (20130101); G01S 19/25 (20130101) |
Current International
Class: |
G01S
1/02 (20060101) |
Field of
Search: |
;342/357.01,357.06,357.09,357.12,357.13 ;701/213,215 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Phan; Dao L
Attorney, Agent or Firm: Law Offices of Thomas E. Schatzel,
A PC
Claims
What is claimed is:
1. A mobile GPS-aiding system, comprising: a GPS reference receiver
to collect GPS navigation messages; a GPS-aiding data network
server to distribute over the Internet ephemeris and almanac data
gleaned from said navigation messages; a number of commercial
broadcast radio stations to publish such ephemeris and almanac data
on particular sub-carriers; a number of vehicles equipped to
receive radio broadcasts and said sub-carriers and to retransmit
them locally via Bluetooth; at least one mobile, portable GPS
receiver, for operation near any of the vehicles, and able to
receive Bluetooth transmissions with the ephemeris and almanac
data, together with the identity of the radio broadcast station
then being tuned; and a breadcrumb database to index the locations
of the radio broadcast stations.
2. The mobile GPS-aiding system of claim 1, wherein each mobile,
portable GPS receiver may contribute to the breadcrumb database
after a location fix is computed.
3. The mobile GPS-aiding system of claim 1, wherein if the location
of a particular radio broadcast station is already known to the
breadcrumb database, then the location is accessed and used to get
a head-start on finding a position solution by the mobile, portable
GPS receiver.
4. A mobile GPS-aiding system vehicle, comprising: at least one of
an automobile, truck, motorcycle, trailer, bus, train, airplane,
ship, barge, trolley, or other mobile vehicular transport; a
broadcast radio receiver carried aboard the mobile transport to
receive commercial radio broadcasts with sub-carriers modulated
with GPS aiding information obtained from a GPS reference receiver;
a piconet connected to the broadcast radio receiver and providing
for a local retransmission of said GPS aiding information via
Bluetooth wireless links; wherein, the locations of any
transmitters tuned by the broadcast radio receiver are logged into
a breadcrumb database for inclusion in said GPS aiding
information.
5. The vehicle of claim 4, further comprising a subscription to a
service provider having: a GPS reference receiver to collect GPS
navigation messages; a GPS-aiding data network server to distribute
over the Internet ephemeris and almanac data gleaned from said
navigation messages; subscribers including a number of commercial
broadcast radio stations to publish such ephemeris and almanac data
on particular sub-carriers; end-users including at least one
mobile, portable GPS receiver, for operation near any of the
vehicles, and able to receive Bluetooth transmissions with the
ephemeris and almanac data, together with the identity of the radio
broadcast station then being tuned; and a breadcrumb database to
index the locations of the radio broadcast stations.
6. A subscription service, comprising: a GPS reference receiver to
collect GPS navigation messages directly from orbiting GPS
navigation satellites; a GPS-aiding data network server to
distribute over the Internet GPS system ephemeris and almanac data
interpreted from said navigation messages; subscribers including a
number of commercial broadcast radio stations to publish such
ephemeris and almanac data on particular sub-carriers; end-users
including at least one mobile, portable GPS receiver, for operation
near any of the vehicles, and able to receive Bluetooth
transmissions with the ephemeris and almanac data, together with
the identity of the radio broadcast station then being tuned; and a
breadcrumb database to index the locations of the radio broadcast
stations.
7. A portable client, comprising: a piconet transceiver for
obtaining GPS aiding information and breadcrumb information relayed
from a network server and a GPS reference station over a broadcast
radio receiver and a piconet server within range of their included
Bluetooth wireless links; and a navigation receiver for computing
and outputting fully autonomous position solutions from pseudorange
signals it receives directly from orbiting navigation satellites
and that are assisted initially by said GPS aiding information and
breadcrumb information provided by the piconet transceiver;
wherein, a position solution for a nearby landmark is earlier
associated in a database of said breadcrumb information with an ID
code as an index that will provide an initial position estimate of
the navigation receiver when operating within said a corresponding
service area of said broadcast radio receiver.
8. A navigation satellite receiver system, comprising: a portable
client comprising a navigation receiver and a piconet transceiver;
a piconet server mounted to a vehicle and accessible to the
portable client by a wireless connection in a local area supported
by the piconet transceiver; and a breadcrumb database associated
with an Internet server in communication with the vehicle via
commercial radio broadcasts, and that accumulates and distributes
positions corresponding to the commercial broadcast transmitter
locations according to an identifier (ID) recognizable to the
portable client.
9. The system of claim 8, wherein: the portable client includes a
device to fetch a position from the breadcrumb database to improve
its ability to obtain a position solution.
10. The system of claim 8, wherein: the portable client includes a
device to contribute a position to the breadcrumb database for
later visitors to the same commercial broadcast service area to
improve their ability to obtain a position solution.
11. The system of claim 8, wherein: the portable client includes a
device to select a landmark name from the breadcrumb database and
then receive a position from the database to improve its ability to
obtain a position solution.
12. The system of claim 8, wherein the portable client includes a
navigation receiver process for extending its fix range under weak
signal conditions, comprising: a device for receiving a GPS
navigation message that has data bit wavelength and a half cycle
ambiguity; a device for accepting parity errors in said NAVdata GPS
navigation message that otherwise prevent its decoding; a device
for extracting a data bit phase from said NAVdata message; a device
for estimating the number of data bits N.sub.set by equating a
predicted range to a measured range when a time-stamp is not
available, but a measured bit transition time (BTT.sub.m) and
codephase are available; and a device for outputting a position
solution.
13. The system of claim 12, wherein: the device for receiving
depends on the locally generated frequency to be within half of the
GPS navigation message bit rate of the true frequency in order to
observe GPS navigation message data-bit phase reversals.
14. The system of claim 13, further comprising: a device for using
a high sensitivity code and frequency tracking loop driven by a
strongest signal from a search window with a multitude of codephase
and frequencies hypotheses from long non-coherent integrations
times, and such that the frequency spacing between hypotheses is
small enough to reduce the frequency error, and the inputs to a bit
transition time (BTT) estimator are the consecutive in-phase and
quadrature correlation results at the code and frequency loop
state.
15. The system of claim 12, wherein: the device for receiving is
such that histogram with several elements is used to integrate
counts of events when the dot product at a given bit transition
time (BTT) hypothesis is negative, and the BTT is declared found
when the histogram count at a particular BTT hypothesis reaches a
confidence value above the other candidates.
16. The system of claim 12, wherein: the step of device for
receiving is such that a bit transition time (BTT) location is
converted to GPS time by associating a BTT location with the sum of
the codephase plus a GPS-millisecond counter adjacent to an epoch
location of a winning BTT histogram.
17. The system of claim 12, wherein: the device for receiving is
such that after a bit transition time (BTT) is determined, the GPS
navigation message is demodulated by forming another dot product
test statistic at a best hypothesis of a data bit phase.
Description
1. FIELD OF THE INVENTION
The present invention relates to the aiding of satellite
positioning system receivers with navigation message data and rough
location information, and more particularly to using commercial
radio broadcasts and their matching mobile vehicle radio receivers
to provide GPS-aiding information in piconets around each of the
many vehicles so equipped.
2. DESCRIPTION OF THE PRIOR ART
Mobile, portable GPS receivers are often turned on after many hours
or days of being turned off. When they are turned back on, they can
be in a very different place than the last position fix. So such
mobile, portable GPS receivers can produce much quicker initial
position solutions, and save precious battery power, if they are
spoon-fed fresh almanac and ephemeris data, and a rough idea of
where they are. If the rough location information is good to within
half of the pseudorandom noise (PRN) codephase propagation
distance, e.g., 150-km, then the integer ambiguities in the z-count
do not have to be solved immediately, and the GPS-millisecond
system-time will be known. The initial carrier frequency search can
then proceed quicker because the right satellites to search for
will be known from the start, and the correct Dopplers can be
assumed.
Such externally supplied aiding information is conventionally being
supplied by mobile telephones. So-called assisted-GPS (A-GPS)
receiver technology allows the mobile phone infrastructure to track
the locations of GPS handsets, e.g., for Federal e-911 rule
compliance. A-GPS can also be used make position solutions possible
under more difficult satellite signal level conditions. The
cellular network signal will typically be very strong in a small
region around a cellular base station so a high
signal-to-noise-ratio (SNR) can be guaranteed for reliable low
bit-error-rate reception. Good signals can support high-data-rate
signals for voice, Internet, or data services.
A conventional location positioning session starts with a request
made by the mobile phone that is sent to the cellular
infrastructure. The infrastructure can assume an approximate
location for the A-GPS receiver because it will be close to a
cellular base station with a known, fixed location. The data
communications channel itself is used to communicate important
satellite data which has been continuously collected by another GPS
receiver beforehand. So the need to demodulate the navigation
message data from the satellite itself is eliminated.
Some A-GPS devices take advantage of information obtainable from a
cellular based infrastructure that would be useful in a GPS
receiver. For example, an A-GPS receiver can be low cost and simple
because it shares hardware and information between the cellular
communication system and GPS receiver. It is not considered an
autonomous satellite positioning system (SPS) receiver because the
hardware can only measure the fractional part of the total range.
It does not demodulate the SPS data message which includes the
timing information needed to form a total pseudorange. Instead, it
uses an estimated total pseudorange based on the known nearby cell
station position. So it can only compute a relative position that
is within a circle of the true position. Such has a radius that is
one half the theoretical maximum fractional range. In the GPS case
for example, the 1023-chip PRN sequence is one millisecond long,
which is a ranging distance of roughly 300-km. So the working range
is roughly .+-.150-km around the approximate location.
If the GPS receiver's clock offset from GPS time is not known, then
the fractional range measurement is referred to as a fractional
pseudorange because it contains the sub-millisecond portion of such
clock offset. When the clock uncertainty grows to
.+-.0.5-millisecond, then the relative positioning working range is
reduced to only .+-.75-km because of an ambiguity of whether the
addition of the clock bias rolls the fractional pseudorange by plus
or minus one millisecond. Such effect is different on each
satellite and occasionally it is impossible to resolve if the
position error exceeds 75-km. However, in both cases of whether the
measurement is fractional range or fractional pseudorange, the
75-km working range is more than the range to typical coverage of
cellular base station.
Finding the codephase of the satellite transmissions requires the
mobile GPS receiver hardware to test a range of hypotheses of the
code phase and frequency of the pseudorandom noise (PRN) signal.
Knowing approximate location and time, as well as having an
accurate frequency reference and a way to predict the nominal
satellite Doppler, will greatly reduce the number of codephase and
frequency hypotheses needed.
A smaller search box means more time can be spent at each
hypothesis. The luxury of being able to spend more time can be used
to improve the signal-to-noise ratio (SNR). An improved SNR allows
the signal to be found in more demanding conditions. Thus at each
code and frequency hypothesis, an A-GPS receiver sums the in-phase
and quadrature components of the down-converted signal before
squaring to form power, and then sums power after squaring. This is
called coherent integration followed by non-coherent integration.
The variance of these sums decrease with integration time, allowing
a power that is above the noise power average to be detected. The
standard deviation plus noise average drops below the signal
power.
Taylor, et al., U.S. Pat. No. 4,445,118, issued in 1984 (Taylor
'118), describe aiding a GPS receiver with an approximate position
of a nearby transmitter, e.g., a radio tower or satellite. A
benefit of providing such externally sourced information is a
faster acquisition time because of the improved starting guess of
the satellite Doppler observed at this location. Taylor '118
teaches transmitting the information at a carrier frequency similar
to the SPS satellite frequency so both signals can be tuned by the
same receiver hardware.
Krasner, in U.S. Pat. No. 6,208,290, (Krasner '290) describes a
similar position aiding scheme. A cell-based communication system
has a GPS receiver effectively embedded into a cellular network.
The aiding improvement is similar to that taught by Taylor
'118.
A cell-based information source, like that described by Krasner
'290, gets its aiding information from the cell itself as the data
source. Krasner '290 describes a cellular network infrastructure
with cell sites and cellular service areas supplemented by a GPS
Server. Such is directly connected to the cellular switching
center, the land based network, and a query terminal. In this
system, a request for service, as a result of an emergency e-911
position request or other service request, enters the network
according to the number of cellular mobile subscriber. The response
is processed from inside the cellular network infrastructure, which
is closed to the general public.
The location determination of the cell base stations or cellular
service areas themselves is not specified directly, but it is
implied and logical that those positions are determined according
to the actual physical layout of the network. For normal operation,
all sites are known because they have been keyed into the cell base
information source, and thus, can be assumed to be known by the
cellular operator. Krasner '290 defines a cell base information
source as the cellular communication infrastructure with an
embedded GPS server. In this setting, a request for position
migrates through different parts of the system so that the
approximate position that assists in the position determination
comes from inside the cellular network. In effect, Taylor '118
applies to providing information from the point of view of cellular
network provider which has access to all parts of the
infrastructure, and thus can exploit the characteristics of such a
network.
Krasner '290 assumes the client has access to an accurate database
for most all cellular base stations at all times of product
operation and life.
Demodulating the 50-bits-per-second GPS navigation data message
(NAVdata) requires specialized hardware and software capabilities.
After finding the signal in the larger search, early-punctual- and
late code hypotheses are centered at the best codephase and
frequency hypothesis for both the in-phase and quadrature channels
of the down-converted signal. Early and late correlators are used
to drive a code tracking loop that pushes the punctual code to the
top of the autocorrelation peak, e.g., to get maximum signal power
and the best estimate of the codephase. The punctual correlators
are used to form a frequency error discriminator that eases the
frequency error towards zero. The 180-degree phase shifts of the
carrier caused by the bi-phase modulation of the NAVdata can then
be observed in the frequency error discriminator.
The Qualcomm A-GPS receiver is implemented in a Mobile Station
Based method (MS-based) that receives a starting position of the
nearest cell site and the ephemeris for its visible satellites, and
a Mobile Station Assisted (MS-Assist) positioning mode that
receives the visible satellites and their expected Doppler. In
MS-based, the position is computed in the receiver. In MS-Assisted,
the fractional pseudoranges are returned to the network and the
position is computed inside the cellular network. In both cases,
search time is improved when the Doppler is computed at from the
approximate location.
In the A-GPS receiver, the SPS satellite information is not
collected in the Mobile Station SPS receiver. Such is both for
simplicity and also because the collection is not required with
such a tight integration of the SPS receiver and the communication
receiver. The SPS satellite data message can always be collected at
a remote location connect to the cell-based information source so
that satellite position data is available for either method. Also,
since the cell station is always within 75-km, additional timing is
not needed to form total pseudorange.
The A-GPS receiver derives its time and frequency directly from the
cell-based infrastructure. For example, the local oscillator for
the SPS signal downconversion and sampling is synthesized directly
from the oscillator used to downconvert and lock the to the
communication signal from the cellular base station signal. Such
removes the need for a separate SPS oscillator and also improves
knowledge of the frequency reference for the SPS receiver when the
cell base station clock is itself synchronized to an accurate time
and frequency standard.
Time is derived from data messages in the cellular communication
signal protocol, and can be improved using additional
round-trip-time propagation measurements that can be made between
the base station and the mobile station.
A-GPS receivers that rely on the cell-based communication receiver
downconverter oscillator cannot operate unless the communication
receiver is on and is also locked to the base station. Thus a
position request requires a handshake with the communication
receiver to take it out of sleep mode if it currently in that
mode.
The A-GPS receiver is intimately tied to the cellular network and
cell-based information source for normal operation. The location
services that the network can provide are only enabled by the
operator of the cellular network for authorized clients who both
have the required A-GPS hardware and are also subscribers to that
service. The implication from the embodiments and drawing
demonstrate that a position request ripples through many parts of
the system. The cell-based information source is not available to
non-subscribers.
By contrast, the conventional SPS receiver has its own low cost
oscillator and time source, such as a local 32-kHz low power
oscillator, and can only determine its time and frequency
information by receiving timing information from the SPS
satellites. In addition, the conventional SPS receiver must contain
hardware that can observe the SPS carrier frequency phase so that
data message and timing message on the SPS carrier can be
demodulated. Otherwise it has no way to obtain the time data and
satellite position data.
The conventional SPS receiver can compute an autonomous fix because
it can measure total pseudorange by observing the local time of the
reception of transmission timing data in the SPS data message. The
total pseudorange is measured as the difference between the
received and transmitted time stamp where the received time stamp
is the local millisecond time of the received data bits plus the
phase of local PRN code sequence where maximum correlation is
observed, e.g., the fractional pseudorange. The transmitted time
stamp is the value in the data message. Such capability allows the
position to be calculated without a known starting reference point.
In other words, it can start its searching and position estimation
at the any position, such as the center of the Earth. The
autonomous receiver has an advantage of being able to fix
independent of aiding information. The A-GPS receiver can only fix
relative to a known location provided by the cell-based information
source.
The cell site position information is not generally available even
when using the cellular signal. Currently, most cellular operators
closely protect the cell site position information for competitive
and business reasons. Access to the cell-based information source
described by Krasner '290 is only available through deep
integration of the SPS receiver hardware into the mobile phone and
cell-based network infrastructure.
What is needed is a broader, more global and penetrating way to
massively provide GPS aiding data even where cell phone support is
weak or missing altogether.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
system for GPS-aiding data for mobile GPS receivers operating near
mobile-vehicle commercial-broadcast receivers.
It is another object of the present invention to provide a
satellite positioning system receiver that can obtain and use
GPS-aiding data from nearby mobile aiding receivers.
It is a still further object of the present invention to provide a
satellite positioning system receiver that does not depend on a
cellular telephone network to generate or carry GPS-aiding
information.
Briefly, a mobile GPS-aiding system embodiment of the present
invention comprises a GPS reference receiver to collect GPS
navigation messages, a GPS-aiding data network server to distribute
over the Internet ephemeris and almanac data gleaned from the
navigation messages, a number of broadcast radio stations to
broadcast such ephemeris and almanac data on particular
sub-carriers, a number of vehicles equipped to receive the radio
broadcasts and the sub-carriers and to retransmit them locally via
Bluetooth. And mobile, portable GPS receivers operated near any of
the vehicles and able to receive Bluetooth transmissions with the
ephemeris and almanac data, together with the identity of the radio
broadcast station then being tuned. A database is used to index the
locations of the radio broadcast stations. Each mobile, portable
GPS receiver contributes to such database after it computes a
location fix. If the location of the radio broadcast station is
already known to the database, then the location is accessed and
used before finding a position solution by the mobile, portable GPS
receiver.
An advantage of the present invention is that GPS aiding
information is provided in ad-hoc Bluetooth piconets associated
with cars, truck, boats, and other vehicles tuned to receive
commercial radio broadcasts.
These and other objects and advantages of the present invention
will no doubt become obvious to those of ordinary skill in the art
after having read the following detailed description of the
preferred embodiments which are illustrated in the various drawing
figures.
IN THE DRAWINGS
FIG. 1 is a functional block diagram of a system embodiment of the
present invention;
FIG. 2 is a functional block diagram of a vehicle embodiment of the
present invention;
FIG. 3 is a functional block diagram of a portable navigation
receiver embodiment of the present invention;
FIG. 4 is a flowchart diagram of a first method embodiment of the
present invention useful in the mobile clients of the system of
FIG. 1; and
FIG. 5 is a flowchart diagram of a second method embodiment of the
present invention useful in the mobile clients of the system of
FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 represents a mobile GPS-aiding system embodiment of the
present invention, and is referred to herein by the general
reference numeral 100. System 100, in one instance, comprises a GPS
reference receiver 102 located anywhere to collect GPS navigation
(NAVdata) messages 104 from constellations of orbiting navigation
satellites 106. Each NAVdata message 104 comprises 1500-bits
broadcast by each satellite at 50-bps on both the L1 and L2 carrier
frequencies. Such broadcast includes GPS system time, clock
correction parameters, ionospheric delay model parameters, the
satellite's ephemeris, and it's health. The information is used to
process GPS signals to obtain user position and velocity. It is
also used when processing precise surveying data.
A GPS-aiding data network server 108 is subscribed to, e.g., by
paid users in a business method embodiment of the present
invention. Users on the Internet 109 are provided with
distillations and repackaged ephemeris and almanac data 110, 112,
gleaned from the navigation messages 104, e.g., and transported by
TCP/IP, GPRS, IP, etc. A number of commercial broadcast radio
stations 112, 114 are opportunistically used to publish such
ephemeris and almanac data on particular sub-carriers 116, 118,
e.g., FM radio broadcasts with radio data system (RDS), digital
audio broadcasting (DAB), DBM, etc. The FM radio broadcasts are
respectively limited to service areas 120, 122. Such practical
limitations allow a mobile GPS receiver 124 operating within these
service areas to assume it is within one-half of the PRN codephase
propagation distance of the corresponding commercial broadcast
radio station 112, 114. This is important because any GPS receiver
operating within the service area 120, 122, will probably be the
same integer number of codephase counts (the z-count) away from the
satellite as is the local radio station 112, 114. Only the
fractional codephase needs to be measured to find the corresponding
pseudorange.
If the location of such corresponding commercial broadcast radio
station 112, 114, has been previously logged into a breadcrumb
database 126, then accessing that database will allow the mobile
GPS receiver 124 to skip solving the integer ambiguities to find a
working z-count and integer millisecond. If the location of such
corresponding commercial broadcast radio station 112, 114, has not
before been logged into the breadcrumb database 126, then the
mobile GPS receiver 124 will contribute such data after it goes to
the lengthy trouble of finding a position solution on its own. Such
breadcrumb referred to herein is reminiscent of the Hansel and
Gretel fairytale story where the children dropped breadcrumbs to
help find their way back home.
A number of vehicles 130-132 are equipped to receive commercial
radio broadcasts, e.g., with sub-carriers 116 and 118. These
specially equipped vehicles 130-132 have matching FM radio
receivers that can decode the sub-carriers 116 and 118, and are
used to haul the NAVdata 110, 112. These FM radios are further able
to retransmit the information via Bluetooth piconets locally to the
mobile GPS 124. (For the Bluetooth Specification, goto,
www.bluetooth.com/bluetooth/.)
Each piconet allows visiting portable devices to ad-hoc connect via
Bluetooth technology to a master, e.g., the vehicle 132. For
example, the 2007 Mercedes-Benz S550 cars are equipped standard
with similar Bluetooth piconets. A piconet starts with two
connected devices, such as a portable PC and a mobile phone.
Bluetooth devices are peer units and have identical
implementations. But, when establishing a piconet, one unit will
act as a master for synchronization purposes, and the other units
will be slaves for the duration of the piconet connection.
Such mobile GPS 124 can be integrated with the AM/FM radio,
cellphone, satellite radio, and a navigation system, e.g., into the
Mercedes-Benz COMAND system. The operational result is mobile GPS
124 that can initialize quicker and operate in areas with very
faint GPS signal levels, such as commonly occur in heavy forests
and cities with tall buildings.
Other mobile, portable GPS receivers 134, 136, operated near any of
the vehicles 130-131, may be able to receive Bluetooth
transmissions with the aiding information.
Vehicle 131 is represented here as not presently receiving FM
transmissions, but it could have nevertheless stored the ephemeris
and almanac data, together with the identity of the radio broadcast
station tuned. If the data is not too stale, it could supply
Bluetooth transmissions of aiding data and supply its own computed
position solution for a breadcrumb to near-enough mobile GPS units
124, 134, 136, etc. An ad-hoc Bluetooth link mechanism is used to
make the connections as the opportunities arise.
The breadcrumb database 126 is used to index the locations of the
radio broadcast stations. Each mobile, portable GPS receiver may
contribute to such database after it computes a location fix, e.g.,
via GPRS on a cellphone network. If the location of the radio
broadcast station is already known to the database 126, then the
breadcrumb location can be accessed and used to speed up the
finding of a position solution by the mobile, portable GPS receiver
itself.
FIG. 2 represents a mobile embodiment of the present invention, and
is referred to herein by the general reference numeral 200. Mobile
200 includes a vehicle 202 such as a car, truck, bus, train,
airplane, ship, etc. Attached to, carried by, and powered by the
vehicle 202 are a commercial broadcast receiver 204, and/or a paid
subscription satellite radio receiver 206, and a wireless piconet
208. For example, the radio receiver 204 can be an AM/FM type able
to support and read data modulated on subcarriers or subchannels,
the satellite radio receiver 206 can be a SIRIUS brand popular on
new cars sold in the US, and the wireless piconet 208 can include
BLUETOOTH devices. In operation, radio receivers 204 and/or 206
demodulate GPS aiding data that was obtained by a GPS reference
station, as in FIG. 1. Such aiding data includes NAVdata message
information that may be hard for a local GPS receiver to obtain on
its own because it just now is initializing, or GPS signal
conditions are weak. The aiding data does not go stale immediately,
in fact, some data stays valid and useful for relatively long
periods of time. Therefore, the wireless piconet 208 is able to
store useful information for local GPS receivers and provide it
even when there has been a period of time since it actually
received fresh aiding information. Such aiding data can include
rough estimates of present location, system time, and z-count data.
They also include breadcrumb information for the local area if any
is available.
FIG. 3 represents a portable client embodiment of the present
invention, and is referred to herein by the general reference
numeral 300. Client 300 includes a wireless piconet 302 for
receiving GPS aiding data, and a GPS navigation receiver 304.
Alternatively, the client 300 may further include a cellular
telephone 306 able to communicate with GPRS, GSM, etc., to reach
the cellular and telephone networks, and the Internet. Such
portable client 300 can receive breadcrumb and GPS aiding
information ad-hoc over a BLUETOOTH link with any nearby provider,
e.g., mobile 200 (FIG. 2). It can update breadcrumb databases, such
as database 126 (FIG. 1), e.g., via cellular telephone 306.
Mobile clients 124, 134, 136, 300, search for satellite signals
using an approximate position with the smallest uncertainty. Each
mobile client applies pre-fix procedures to prepare the
measurements before computing. A position with different
sensitivity levels is computed according to the uncertainty, and
the types of measurements it is able to obtain from the duration of
the position attempt session.
FIG. 4 represents a method embodiment of the present invention, and
is referred to herein by the general reference numeral 400. The
method 400 is implemented as software on mobile clients 124, 134,
136. A step 402 begins with the mobile client trying to obtain an
identification (ID) code from the cell base 103. If an ID is
obtained, it can be used as an index to a database of previously
determined positions. A step 404 checks to see if the need for a
particular ID and its associated aiding information were
anticipated and preloaded in local memory. If not, a regional,
national, or worldwide database needs to be consulted. A step 406
attempts to log-onto the Internet 106 and access server 108. If the
log-on is successful, database 126 is indexed with the ID. If a
mobile client 104 had previously computed a fix, then database 126
will have it to supply the request of mobile client 124, 134, 136.
A step 408 supplies a locally stored aiding information. A step 410
uses the data fetched to seed a signal search. A step 412
determines position, either autonomously or with help. If the
aiding information is accessible, then the step 412 can proceed to
provide a result more quickly and at higher receiver sensitivity.
In any event, a step 414 contributes the position solution obtained
to the database 126 for use later by others, or its own revisit. It
may be worthwhile for a mobile client 124, 134, 136, and 104 to
store all the aiding information locally too it has generated, on
the assumption it is likely to revisit places it has been
before.
At the start of worldwide deployment, the number of known reference
points will initially be small. The volume will grow over time and
as a function of the success of the commercial products. Krasner
'290 assumes the client has access to an accurate database for
almost all cellular base stations at all times of product operation
and life. So a database is not needed as each cell-site is
self-identified.
Typically, a mobile client 124, 134, 136, can communicate with many
cellular base stations 103, the cellular switching system controls
which cellular base station is used at a particular time.
Additional information inside the network can be used to predict
the approximate location more accurately than with a single cell
site in the approach. The strongest signal is not always the
closest, e.g., due to obstructions or because the closest tower is
busy. But this information can nevertheless be used by the
cell-based information system. The actual cell site positions can
be defined more accurately by the cellular infrastructure, since it
knows the exact location of the towers. It can derive an accurate
position from a survey or a map. Other SPS receivers with access to
cell-based information sources use methods for faster and lower
power consumption position session. These take advantage of the
cell site density in producing a more accurate approximate
position.
System 100 can be less accurate in providing the approximate
locations, compared to those obtained directly from an information
system. As a result, it employs a more demanding and complex
relative positioning method to manage a potentially large position
uncertainty. In the Krasner '290 approach, the position uncertainty
is always minimal because it only attempts a fix when it is in
contact with a cellular base station where the position is assumed
to be accurately known. However, because the SPS receiver 102 is
autonomous, and is required to have extended GPS capabilities, any
degradation in the approximate location associated with the cell
site can be managed. The highest sensitivity positioning can be
maintained, as long as it operates where it or other eGPS receivers
have operated in the past.
The system 100 may require stronger signals in a new area to get
its very first fix, but will thereafter improve its sensitivity as
long as it can remember this station-ID and the position it
computed.
Each receiver uses a conservative error growth calculation to
estimate a worst case error in the parameters. For short periods of
off-time, the error grows according to an acceleration model. For
consumer applications, an acceleration a=0.25 to 0.5 g (where
g=9.81 m/s/s) is a reasonable value. Constant acceleration is
reasonable until the velocity growth hits the worst case velocity
(maxUserVel) which is a parameter tuned for the particular
application.
Thus, until maxVel is reached, VelUnc=FixVelUnc+a*dt
Punc=PfixUnc+0.5a*dt*dt Where dt=t(now)-t(last fix)
After maxVel is reached, VelUnc=maxUserVel
PosUnc=Punc(maxVel)+maxUserVel*dt Where Punc(maxVel) is the value
of the position uncertainty when the maximum velocity was
calculated.
A similar model is made for the time and drift uncertainty,
DriftUnc=DriftFixUnc+driftRate*dt BiasUnc=BiasFixUnc+integral
(DriftUnc) Where driftRate is a function of the stability of the
oscillator with time, and the time uncertainty integrated this
value. If a temperature sensor and a drift verse temperature model
are available, the error growth rate for the drift and bias
uncertainties can be modeled.
Data in a worldwide reference point database 126 is accumulated
over time in a business model embodiment of the present invention.
By sharing computed positions and station-IDentities (ID's) with
the position registration server 108, a worldwide set of reference
points can be constructed. Such provides each mobile client 124,
134, 136, with high sensitivity position capability in any cellular
network where a station-ID is readable in the available in the
unprotected communication protocol. Other forms of identifying the
radio station 112, 114, could be acquired or registered.
Initially the position registration server 108 starts with no
database of reference points, e.g., breadcrumbs. Such record a
place that has been encountered before. When product shipments
begin to new clients, the receivers will have an empty database
126. However over time, such mobile clients 124, 134, 136, compute
autonomous fixes and begin to communicate periodically with the
position registration server 108.
As part of a setup procedure or a pre-travel preparation, a client
receiver can connect to the position registration server 108 via
the Internet. A simple selection process is used to request
reference points for places it intends to operate. Such as places
near a primary residence or places it intends to visit. In the
early phase of a product roll-out, such requests will not yield a
great number of points in response. However, over time, as the
number of autonomous fixes performed in the mobile client 104 in
the field grows, and more points of interest are defined, the
position registration server 108 and database 126 will also grow to
be quite extensive.
As a business model method to grow the position registration
database 126 faster, a contest can be used that motivates clients
to operate their GPS receivers in conditions that allow reliable
autonomous location determination and then to log-on periodically
to the position registration server 108 to share their points. They
are rewarded more for being the first client to operate in a new
region and provide the information to the position registration
server 108. They are rewarded proportionally less when they provide
information for points already supplied but yet still help to
improve the accuracy of an aggregate position supplied by other
contestants for a given station-IDentity or breadcrumb. Once that
position estimate has converged to a satisfactory non-varying
average, the contest for that station-IDentity or breadcrumb is
closed.
Thus, just as a mobile phone can store phone numbers, it can store
breadcrumbs of places it wants to go or has been. Algorithms for
minimizing the number of breadcrumbs are implemented in the client
to minimize the amount of memory required. For example, a single
position may have a long list of station-IDentifiers that are
within a tolerable range of the position so that the breadcrumb
position is within 75 or 150-km of the true position. The goal is
to store one position that contains the most station-ID's within
that region. Such is common when there are many Station-ID's coming
from the same cellular base station. If the client reads a new
station-ID a short time after computing a fix, it can associated
the new station-ID with the previous fix and also create an
uncertainty for this station-ID which is the uncertainty of the
previous fix plus the uncertainty growth due to receiver movement
since the fix. In this way, the receiver does not have to be fixing
continuously, but the modem can continue to interrogate for new
station-ID's.
Similarly, the station-ID's may not always be completely unique in
a worldwide sense. The position registration server 108 hosts
algorithms that manage the master database 126. It is possible to
have more than one of the same ID's, but generally they will be in
different geographic regions.
If a client enters an area for which is has no breadcrumbs, it may
be able to do an autonomous position and therefore create a new
breadcrumb at the current station-ID that it can subsequently share
this with the position registration server 108. If it is not
possible to fix, it can send the current station-ID to the position
registration server 108. It also sends along its most recent
position, time tag, station-ID, and the current configuration for
the maximum velocity it can experience in normal operation. If the
server 108 has that ID and it is unique, then the server can
confidently send the position for this station-ID to the client 102
along with a configurable number of additional breadcrumbs around
this area. The number depends on the time till next communication.
If the client requests often, only a few breadcrumbs around the
current one are needed. If the client requests rarely, a larger
reply may be sent. In a constant-on operation, the server could
send the points surrounding the current location and the client can
continuously trim the previous breadcrumbs to minimize the
storage.
If the ID is not unique, the server can check the position for
reasonableness of being in proximity of the multiple positions in
memory for this ID. The maximum distance that can be traveled since
the last fix can be calculated produces a circle around the last
position. As an even more conservative check, the speed of the
fastest executive jet can be used to predict the maximum range for
the last position. If this circle can be used to identify the
obvious position for the redundant station-ID, then the client is
sent the breadcrumb. However, if there is ambiguity, the server
will not reply with a position and the mobile client 124, 134, 136,
will be required to do an autonomous fix or wait for the signal
conditions to improve.
An ambiguity between two possible station-ID locations can be
resolved by looking at the last fix and deciding which is nearest.
The nearest is most likely correct.
DelPos=maxVel*(timeNow-timeFixAtCellID1)
Location area codes change much slower. The range covered by an
location area code is much larger. In one case, a location area
code was valid for 186-km=115-miles. The one sided range is about
58 miles or 93-km.
In embodiments of the present invention, the approximate position
in eGPS system never comes from the cell-based information source,
e.g., in contrast with the cellular network described in Krasner
'290. Instead, it comes via the Internet from server 108 that is
independent of cellular network provider. It cannot access the
cell-based information source known to the cellular network only.
Furthermore, the position database is grown by the customer
operation, not by the cellular network provider.
The preferred SPS receiver has many of the characteristics of a
conventional receiver in that it can obtain all the timing
information in the SPS data message. Like a conventional receiver,
it can fix autonomously with no aiding information using the timing
information in the data message to form a total pseudorange
measurement. It also has the high sensitivity of an A-GPS receiver
and can still extract the codephase below where the signal is too
weak to extract any larger wavelength timing information from the
signal. With the wavelength of the codephase at one millisecond,
there is only .+-.0.5-millisecond (.+-.150-km) of measurement
observability of the total pseudorange which determines how far it
can move the position estimate from the starting value. The
extended GPS (eGPS) receiver can extend its fix range far beyond
the 150-km radius in signal conditions where it is able to also
extract the data bit phase of the data message, even when it might
not be able to decode the data bits themselves without parity
errors. With GPS, the 20-milliseconds wavelength of the data bit
produces a half cycle ambiguity of .+-.10-millisecond. With this
larger wavelength measurement, the range of position in weak
conditions can be extended from the range of the codephase at
150-km out to 1500-km (940-miles) and beyond that from an
approximate position.
Using breadcrumbs or reference points, the eGPS receiver bounds its
starting position uncertainty and maintains high sensitivity fixing
whenever it returns to an area where it has previously computed a
position and can associate that position with an unique identifier
that can be communicated automatically or by human intervention.
The eGPS receiver builds its own database of reference points.
Bread crumbs are collected at the position registration server 108
where they then can be shared with other eGPS receivers. Cellular
communications cellular base stations are one such Type of
reference point since a cellular modem can generally read the
station-IDentity in the normal communication protocols without
using the cellular based information source.
A method of operating the mobile client 124, 134, 136, and 104 for
extending its capabilities is firstly to obtain the most timing
information possible from the SPS signals, even in weak signal
conditions, in order to fix autonomously with the highest level of
uncertainty in its starting position. Secondly, when it is able to
fix, it updates a database of fresh reference points and
identities. These are thereafter used as approximate starting
locations on subsequent fix sessions started in the vicinity of the
same reference point.
FIG. 5 illustrates such a method embodiment of the present
invention, and is referred to herein by the general reference
numeral 500. The method 500 attempts to receive and demodulate the
NAVdata message in a step 502. A step 504 accepts any parity errors
that may be occurring, e.g., due to weak signal conditions. A step
506 extracts the data bit phase. A step 508 sees if the BTTm and
codephase are nevertheless available. If they are, a step 510
estimates the number of bits Nset by equating predicted and
measured ranges. Position solutions can then be produced. A step
512 updates the database, e.g., breadcrumb database 126 (FIG.
1)).
The receiver should have a millisecond clock used as the reference
for the codephase measurement. Its time error can be expressed as
the sum of an integer error in the time of the millisecond event
and a sub-millisecond offset between the receiver millisecond event
and the true SPS millisecond time. The bias is the true time
(.+-.0.5-millisecond) difference between the receiver millisecond
counter and SPS time. The true range (Rtotal) is the distance
between a satellite and a receiver, also expressed in integer and
fractional-milliseconds (Rint, Rfrac). The choice of using the
millisecond event as the main time step comes from the GPS case
where the satellite pseudo-random-noise sequence (PRN) repeats
every millisecond. The codephase is a positive number between zero
and one millisecond as the time between the received PRN epoch and
the SPS receiver millisecond event prior to the received epoch
(defined as msecEpoch).
If Rtotal is known, one equation can update two variables because
bias and msecAdjust can be solved for by isolating the integer and
fractional components. Summarizing, biasEstimate=fractional
part{SpStransmitTime+Rtotal-msecEpoch-codephase}msecAdjust=integer
part {SPStransmitTime+Rtotal-msecEpoch-codephase}.
MsecAdjust and bias can be estimated even if an approximate
position only is available. A resulting estimation error is
proportional to the error in estimated range to the satellite. The
error is introduced in the estimated range (Rapprox) and it
produces exactly the same amount of time set error in the
adjustment variables msecEpoch and biasEstimate. The amount of
actual position error that would create this range error depends on
the geometry.
Even with a clock bias, the error in the time setting of msecAdjust
and the estimated bias is dependent only on the error in
approximate range, which itself is dependent on the error in the
position.
Three types of timing measurement for an SPS satellite can be
defined:
Type-1: total measured pseudorange using the received time message
in SPS data message, data bit phase (BTT), and measured fractional
pseudorange (codephase);
Type-2: data bit phase (BTT), and measured fractional pseudorange:
and
Type-3: measured fractional pseudorange only.
The pseudorange (PR) can also be represented as the sum of an
integer millisecond range and a fractional millisecond range. In
GPS, the PRN repeats every millisecond and a typical value is 60-85
milliseconds. An SPS receiver generates zero codephase when its
local PRN epoch occurs at the same time as the millisecond
clock.
The fractional range is measured as the PRN codephase between zero
and one millisecond where there is maximum correlation between the
locally generated PRN code and the incoming signal. Because the
wavelength of this signal is relatively large, the codephase by
itself can provide an unambiguous relative positioning capability
of .+-.150-km around an approximate position. But only when that
position is within this range of the true position. A full measured
ranging capability is only possible by receiving the SPS satellite
timing message that measures the true integer range. So unlike with
only the codephase, an approximate location is not needed to obtain
a position. For example, (0,0,0) can be used.
In the GPS case, the time message contains the z-count which
provides the GPS time of a known bit in the data sequence. Having
this time reference means the time of any other data bit can be
predicted. So the usage of z-count can express the time of any
specific data bit.
.times..times..times..times..times..times..times..times..times..times..ti-
mes..times..times..times..times..times..times..times..times..times..times.-
.times..times..times. ##EQU00001##
The measurement can also be represented as having an integer and
fractional portion (in units of milliseconds). Pseudorange=integer
msec+codephase (msecs)
When the data bit is 20-milliseconds long, the integer millisecond
(intmsec) can also be written in terms of the number of integer
data bits (N) and the fractional data bits phase (BTT):
intmsec=N*20+BTT
When z-count is available (a Type-1 measurement), Nzcount is the
number of total data bits in the pseudorange constructed from the
received z-count. The z-count is the part of the integer
millisecond range that is multiples of 20. The BTT.sub.m is the
measured sub-20-millisecond data bit phase, and measured codephase
makes the fractional millisecond part: Pseudorange
measured=Nzcount*20+BTT.sub.m+codephase.
When a z-count is not available, but a measured BTT.sub.m and
codephase are available (such as with a Type-2 measurement), then N
can be estimated from an approximate time and position. In this
case, the number of data bits N.sub.set is estimated instead by
equating the predicted range to the measured range. Such is formed
by removing the estimated receiver clock bias from the measured
pseudorange. Codephase=Rfrac-bias. Predicted
range=N.sub.set*20+BTT.sub.m+codephase+biasEstimate
Thus, N.sub.set=round to nearest integer[(predicted
range-BTT.sub.m-codephase-biasEstimate)/20].
N.sub.set is calculated correctly as long as the interaction of the
time error and the position error do not combine to make more than
10-millisecond of error in the estimated pseudorange. The position
error couples through the estimated range error and the time error
manifests in the time-tag of BTT.sub.m.
An ambiguity function is evaluated to determine if the N estimate
can be trusted.
The error in the estimated range and the error in the receiver time
appear with opposite signs. Thus, by inspection, N calculates
correctly when, |ErrorNset-ErrorTimeSet|<10, where errorTimeSet
is the error in the satellite vehicle (SV) time set and ErrorNset
is the error in the N.sub.set process.
The time set error is only a function of error in the predicted
range. The common mode error of the predicted range equals the time
set error when the correct N is calculated. The size of the clock
bias does not affect the time set error or N.sub.set error.
|ErrorNset-ErrorTimeSet| defines whether the N can be calculated
correctly or not.
The integrity of the estimated total pseudorange is determined by
the ambiguity function. When the receiver time is set by a Type-1
measurement, then ambiguity function says to:
Trust N.sub.set if |RangeErrorNset-RangeErrorTimeSet|<10, else,
don't trust N.sub.set.
If the maximum position error is less than 5-millisecond, then the
maximum range error in either of these terms is always less than
5-millisecond, and so the ambiguity function can never be larger
than 10, meaning that N.sub.set always calculates correctly.
When the position error is larger than 5-millisecond, the ambiguity
range depends on the relative geometry between the time-set and
N-set satellites. If they are more normal to each other, there will
be more common error between then, leading to less error in
N.sub.set. Conversely, if they are more orthogonal, the error will
be less common between them, leading to more error in
N.sub.set.
With only one Type-1 measurement for time-set, the best situation
may be when the time-set is the highest elevation since it will
have the least amount of horizontal position error. So, the lower
elevation satellites are able to use up the rest of the
10-millisecond minus the error in the time-set SV. However, in case
there are more than one Type-1 measurement, the best case is when
the Type-2 satellite is more normal to the time set satellite so
that the range difference will be more smaller, yielding a lower
ambiguity result. In case there are enough high elevation SV's, it
is possible that the position error can grow past 10-millisecond
and the ambiguity function can still be less than 10-millisecond,
meaning that the range of position error can be larger than
10-millisecond.
A fix with such a high elevation satellites will generally have a
higher position dilution of precision (PDOP) and error than can be
tolerated for a consumer expectation. The fix is generally accurate
enough to allow the higher sensitivity Type-3 measurements to be
added in a second fix that uses the first fix as the starting
point. Such two step process is referred to as self-aiding.
When the receiver time is maintained by a real time clock (RTC) or
any continuously running oscillator and when there are no Type-1
measurements available, then N.sub.set can still be estimated in a
similar way. The time error cannot be measured directly in this
case so a model of the time uncertainty Tunc is used as an upper
bound on its contribution to the N estimate. However, the ambiguity
function changes to:
Trust N.sub.set if |rangeErrorNset|+|Tunc|<10, trust N.sub.set,
else, don't trust N.sub.set.
The GPS NAVdata message has a rate of 50 bits-per-second. Each bit
is 20-milliseconds long and a new NAVdata bit leaves the satellite
synchronously after 20 PRN's are transmitted. For a typical range
to the satellite of 76-millisecond, there are three total bits of
distance and then a fractional part of a data bit that is sixteen
epochs out of twenty complete.
The process of measuring the BTT involves determining the state of
a 20-millisecond counter that defines the GPS millisecond when the
received data bit changes. The start of the current incomplete data
bit is BTT-milliseconds earlier than the millisecond of the current
epoch, but offset from the millisecond by codephase. Adding this
additional event time extends the range over which position can be
computed from the level provided by codephase, from .+-.150-km up
to .+-.1500-km. A method for determining an accurate estimate of
BTT for a weak signal is needed to gain this extension.
To estimate BTT, a phase reversal test statistic is formed for each
epoch hypothesis from 0-19 for the location of the BTT. The test
statistic is the dot product detector based on a series of
correlator sums of the in-phase and quadrature data. In order to
observe the data bit phase reversals, the locally generated
frequency must be within 25-Hz of the true frequency. In
conventional receivers with strong signals, this is done with an
AFC or PLL loop. For weak signals, eGPS receiver embodiments of the
present invention use a high sensitivity code and frequency
tracking loop. These are driven by a strongest signal from the
search window with a multitude of codephase and frequencies
hypotheses from long non-coherent integrations times. The frequency
spacing between hypotheses is small enough to reduce the frequency
error. The inputs to the BTT estimation algorithm are the
consecutive one millisecond in-phase and quadrature correlation
results at the code and frequency loop state.
Every 20-milliseconds, twenty test statistics for the data bit
phase are evaluated that use the in-phase and quadrature sums from
two consecutive time windows. For the highest sensitivity, the time
windows are each 20-millisecond. If the consecutive sums in each
leg have a different sign, then the product will be negative
indicating a potential data reversal.
A histogram with twenty elements corresponding to different BTT
hypotheses is defined to integrate counts of events when the dot
product at a given BTT hypothesis is negative. BTT is declared
found when the histogram count at a particular BTT hypothesis
reaches a confident value above the other candidates.
The BTT location is then converted to GPS time by associating the
BTT location with the sum of the codephase plus the GPS-millisecond
counter nearest to the epoch location of the winning BTT
histogram.
After the BTT is determined, the NAVdata is demodulated by forming
another dot product test statistic at the best hypothesis of the
data bit phase. The Ibtt(k-1), Ibtt(k), Qbtt(k-1), Qbtt(k) sums are
already formed to continue to update the BTT histogram result. Then
these data are used to identify the actual data reversals of the
NAVdata message itself. Initially, the first data bit for I(k-1),
Q(k-1) is declared arbitrarily to any value: typically 0 is chosen.
Then if the test statistic using I(k), Q(k) is negative, indicating
a reversal, then the second bit is declared a one. Otherwise it is
declared zero. Then the time index is moved by 20-millisecond, so
that Q(k-1)=Q(k), I(k-1)=I(k), and a new set of data is collected
for I(k), Q(k) to get the next bit, and the process is
repeated.
When the signal is weak, the reversals might be declared falsely
because a sign change can occur randomly when the signal is weaker
than the noise. As the signal decreases, the dot product result is
more often wrong due to noise in the correlator results. The 50-bps
data is not done reliably, it wont pass parity.
However, the data bit phase is not a single event like the data, it
keeps occurring, so the BTT histogram has the luxury of being also
to integrate for a long period to average out the noise. As long as
the signal is above the noise average, the histogram will
eventually find the correct BTT when enough data is observed. Of
coarse, the noise will cause the noise floor of the histogram to
grow, but the true location will win if integrated long enough. A
histogram up to 16-seconds can be used so that BTT can be estimated
down to -150 dBm or lower. Typically, the data message can only be
demodulated confidently down to a received signal strength of -145
dBm.
After decoding the raw NAVdata bits, the receiver commences a
process of extracting the real message from the bits. It first
looks for the preamble, which a fixed value of 0x8B in hexadecimal,
or its complement in case the arbitrary starting phase is flipped
from the true phase. After finding preamble, it forms a 30-bit word
and computes the parity of twenty-four data bits and compares them
to the 6-bits received parity bits. If they agree, it decodes the
next 30-bit word as the HOW (hand-over-word) which contains the
17-bit z-count. Such variable gives the exact transmission in GPS
time of the first bit of the next subframe which will occur after
eight more words.
The integer millisecond portion of the total pseudorange is
measured by comparing the observed reception time of the z-count to
the value itself. For example, the receiver time tags the received
bits with respect to its local GPS-millisecond clock. It can then
predict at what time it will receive the first bit of the next
subframe because each bit is 20-milliseconds. The difference in the
predicted reception time and actual transmission time yields the
intmsec range. For example, if the z-count itself says the time
will be 6.0 seconds, and the true range plus the time error is
73.5-millisecond, the observed reception time of will have integer
millisecond counter time tag will be 6073-milliseconds and a
codephase of 0.3-millisecond. The integer millisecond range is
6073-6000=73 and the total measured pseudorange is
73.3-millisecond.
Because z-count can only be observed no faster than every six
seconds, BTT is often used to estimate the sub 20-millisecond
portion of the intmsec because the codephase can roll-over or
roll-under after the lost observation of the z-count. Codephase is
the non-linear truncated part of the total pseudorange than has to
have a value 0-1 millisecond. Roll-over occurs when the codephase
is increasing, and rolls over from a value close to a millisecond
value and to a value near zero. Roll-under is the opposite case
when the value is near zero, and decreases and wraps from a small
value to a value close to a millisecond. However, because a signal
can become weak, and disappear periodically, it may be difficult to
observe z-count again, and thus, correcting the intmsec for
roll-over can be difficult. For this reason, BTT is needed to keep
the absolute part of the intmsec in agreement with the current
codephase. Thus, BTT estimation is a continuous process.
A GPS receiver that starts with no information from a cold start
will set its clock from the first z-count by using a guess of the
integer millisecond around 72-milliseconds since this is the
average intmsec. Such sets the time accuracy to roughly
.+-.13-millisecond.
When the receiver decodes other SV's, then the computed intmsec
will have the a common mode time error due to the error in setting
the receivers local GPS-millisecond clock. However, because it is
common mode, then this super millisecond time error can be
observed. Such allows the clock to be observed and removed in the
position fix.
It is important to define the time set error as having an integer
millisecond part, and a part which is fractional defined to be
.+-.0.5-millisecond. If the receiver position is calculated with a
full PR's estimates from z-counts, then the super millisecond can
be observed in the common mode clock bias solution of the GPS fix.
The GPS millisecond counter is then adjusted to correct the error.
The sub-millisecond portion is estimated in the fix. Most receivers
don't move the millisecond location, but the bias estimate can be
made available so that the exact time of the millisecond is still
known. Because the error is common mode, the bias can be estimated
to the accuracy of the position solution. If the position is
accurate to 10 meters, then the time can also be set to that level,
which is about 30-nanoseconds.
An SPS receiver with a data collection ability can compute is own
position by forming total pseudorange and by receiving the
satellite position information from the satellite signal. It can
linearize the pseudoranges and estimate the position directly
without any starting guess of time and position. The eGPS receiver
also has this capability.
A important input for extended GPS fix operation is the accuracy of
the SPS time clock in the SPS receiver. When the error is bounded,
the range of positioning can be extended.
The mobile client 124, 134, 136, and 104 can be equipped with a
number of methods for time maintenance between fix sessions.
An accurate real-time-clock (RTC) can be included in which a master
clock remains powered while the rest of the receiver is off. In
this mode, any clock bias is integrated from a propagating drift
value, so the sub-millisecond portion of the bias can be maintained
for long periods of time. (See, U.S. Pat. No. 6,714,160.)
A 32-kHz watch-type crystal can be kept powered while the master
clock oscillator is turned off. For precision, the 32-kHz clock is
always read on the receiver msec. To calibrate the RTC, the counter
value is remembered at a specific-millisecond where the GPS time is
known precisely after a fix. Then to use the RTC after the master
clock power is restored, the counter is again read at a
new-millisecond, and the difference in the counter values between
the calibration step is added to the previous calibration time to
obtain the current time. Both the super-msec time and sub-msec are
estimated in this step. The advantage of this approach is that it
requires less power than with the higher frequency master clock
oscillator. The rate at which the error grows is mainly a function
of temperature since this causes the frequency to change and the
time error is the integral of the frequency error. It could be
several hours or more before the time uncertainty grows past
5-millisecond. Uncertainties under 50-millisecond might be possible
for a day of off time.
A time interval counter can be built in (See, U.S. Pat. No.
6,473,030). If another device connected to the GPS receiver has
accurate time, then it can send a hardware pulse into the timeDiff
circuit and the timeDiff will produce an accurate measurement of
the time difference between the GPS-millisecond and
external-millisecond. The other device will also send a message to
the GPS of the time of the pulse. This way, after the GPS receiver
has been powered-off, it can be accurately reset from an external
accurate time source. The accuracy is dependent on the aiding
system accuracy. However, a precision on the order of the period of
the master clock is possible. Thus, an error much below a
millisecond is possible.
In another case, the eGPS system is completely off, but time is
maintained elsewhere in the system. Time is then supplied as a
function call rather than as a hardware interrupt. Such is
generally considered less accurate as the message propagation time
is dependent on the host CPU and operating system loading. The
accuracy may not be better than 50-milliseconds.
Alternatively, time can be extracted from a satellites SPS data
message (the z-count for GPS). As shown above, the accuracy is
bounded by the position accuracy when the range is predicted to set
the receiver time.
For each time set model, a time error model is formed that provides
a conservative upper bound on the error magnitude. The source that
is available and has the smallest uncertainty is chosen as the time
source and determines the current time uncertainty (Tunc). The
level of time uncertainty determines how and whether the fix can be
performed.
If the time uncertainty is larger than ten seconds, then a fix is
not attempted.
If the time uncertainty is more than 50-milliseconds and less than
ten seconds, the a special fix method called the no-Z fix (U.S.
Pat. No. 6,714,160) is employed that has an extra unknown to model
the additional error in the linearization of the pseudorange caused
by the satellite position error due to being computed at an
inaccurate time. Thus there are five unknowns rather than the
classic four unknowns of three dimensional position and the clock
bias. The cost of this time error is that an additional independent
measurement is required to estimate the receiver position.
This zone is labeled TU4, where (50-millisecond<Tunc<10
seconds).
If the time uncertainty is less than 50-milliseconds, then the
classic four unknown linearization of the pseudorange is used where
there is not an additional term to account for the satellite
position error.
Assuming four time uncertainties (Tunc) of the time error.
.times..times..times.<<.times..times..times.<<.times..times..-
times.<<.times..times. .times.< ##EQU00002##
The selection of Tu3, Tu2, and Tu1 will be shown to depend the 0.25
and 0.5 wavelength of the 20 millisecond data bit.
Now that the time error zones have been defined, the extended fix
strategy can be presented as a function of the combination of the
time and position uncertainty zone.
Four zones of horizontal position uncertainty are defined around
the most recent position estimate according to the position
uncertainty (Punc). The position can be the most recent computed
fix, or an externally supplied approximate location.
Each position source has a conservative model of how Punc is grown.
The uncertainty from a previous position or breadcrumb grows
according to the maximum user velocity model times the time since
the fix or the indication of the breadcrumb identity. The
uncertainty from a reference point or breadcrumb does not grow
above its normal tracking range uncertainty as long as the
reference identity is still indicated. Otherwise, the uncertainty
grows at the same conservative rate of the maximum user velocity
model. At the start of a session, the source with the smallest
uncertainty is chose as the source of the approximate position.
Also, during a session, new identifiers will also be checked to see
if they provide a smaller uncertainty than the current
uncertainty.
Assuming for relative fix zones based on position uncertainty:
FUZ1--larger than 5-msec (Punc>1500m)
FUZ2--less than 5-msec but more than 0.5-msec
(150-km<Punc<=1500m)
FUZ3--less than 0.5-msec but more than 0.25-msec
(75-km<Punc<=150m)
FUZ4--less than 1/4-msec (Puns<75-km)
In the Fuz1, sub-75-km zone, the position error is known to
.+-.0.25 of a millisecond. Such is maximum position uncertainty at
which the .+-.0.5 millisecond ambiguity in the clock bias can be
observed. A smaller bias uncertainty would mean a larger position
uncertainty can be tolerated. A simple wrap check can be used to
properly un wrap the non-linear wrapping of the receiver clock bias
in the codephase measurement. The codephase with the smallest
absolute value is chosen as a pivot. The codephase to any non-pivot
satellites is adjusted up by one millisecond if the codephase is
less than -0.5-millisecond from the pivot. Similarly, the codephase
is adjusted down by one millisecond if the codephase is more than
0.5-millisecond from the pivot. Such is called the pivot method to
unify the codephases to a common millisecond clock
contribution.
If time can be set using a Type-1 measurement in this zone, then
the time uncertainty must also be less than .+-.0.25 millisecond in
TU1. It is often true that there is at least one satellite with a
less obscured path that can yield a Type-1 measurement. Otherwise,
the time source must be from an RTC or external time source.
In FUZ1, a position can be computed with either the no-Z fix if
Tunc=TU4 or the classic fix if Tunc is lower TU4.
In the Fuz2, sub-150-km zone, the position error is known to
.+-.0.5 of a millisecond. At this level of uncertainty it is not
possible to also resolve .+-.0.5 millisecond ambiguity in the clock
bias with the pivot method used in Fuz1. A method called the grid
search is employed in this region. A grid of candidate positions is
formed around the approximate position with a radius of 150-km
where there is at least one point within a distance of 75-km of the
true position. At this accuracy, the bias can be properly unwrapped
with the pivot method described above. To detect the best grid, a
test statistic is formed that contains the sums of squares of a
double differences of predicted fractional range minus codephase so
that the bias is cancelled. Before squaring, any double differences
larger in magnitude than 0.5-millisecond are adjusted by the proper
+ or - one millisecond to produce a wrap with magnitude less than
0.5-millisecond. Often, a smaller grid spacing helps locate the
best grid position when the geometry is problematic and there are
other nearby minima that are not the global minima but can appear
as the minimum when the grid spacing is large.
After the best grid point is located, the codephases are adjusted
with the pivot method of FUZ1.
If time can be set using a Type-1 measurement, then the time
uncertainty must also be less than .+-.0.5 millisecond and thus in
TU1. Otherwise, the time source must be from an RTC or external
time source.
In FUZ2, a position can be computed with either the no-Z fix if the
Tunc=TU4 or the classic fix if the Tunc is lower TU4.
Experiments demonstrated the how the clock bias effects the ability
to fix with different amount of position error. A simple pivot
method was used to un-wrap the codephase ambiguity. At each grid
point, the difference between the predicted fractional range and
the measured codephase was formed. Then the satellite with the
smallest absolute value was chosen as the pivot. As a way to verify
the correctness of the pivot, the integer millisecond range was
wrapped according to the pivot outcome for each satellite. If the
adjusted integer millisecond was equal to the true integer
millisecond range, then the wrapped sub millisecond linearized
measurement was correct at the grid point. If the adjusted integer
millisecond was wrong, then the position was wrong by a large
amount because the measurement error was then one millisecond or
300-km. With different cases for the clock bias, it was seen that
only the position that has an error less than 75-km
(1/4-millisecond=FUZ1) will properly wrap the codephase minus
fractional range estimate. When the error was larger than 75-km,
but less than 150-km (FUZ2), the bias was not properly unwrapped as
was the case for when the position error was greater than 150-km
(>0.5-millisecond=FUZ3), even when the bias was zero.
The wrapped differences were manipulated to form a test statistic.
For each difference, the next difference was used as a pivot. Then
the difference was adjusted by a millisecond if the absolute value
of the difference was larger than 0.5 millisecond. The test
statistic was the square root of some of squares of the wrapped and
pivoted differences.
The position that was within 75-km always had the smallest
statistic, so the best grid location could be properly identified.
Secondly, grid points that were further than 150-km could provide
an even smaller test statistic that the best, most accurate
position. Such was because the wrapping process allowed the larger
error to be hidden. However, this was easily detected since the
grid point was further than 150-km from the center, and thus,
exceeds the position uncertainty. Such is why it is important that
the Punc estimate be very conservative. If the true error is
larger, then the grid test can fail.
The clock bias wrapping affects the position estimation. If the
bias is allowed to grow beyond the normal definition of
.+-.0.5-millisecond, an interesting phenomenon occurs. In the first
case, the bias was extended to 250,000 m ( -millisecond). The
integer-milliseconds ranges were still correct, but the other
points experienced large errors.
In another example, the bias sign was changed. All the integer
millisecond ranges were wrapped the wrong way, but by the same
amount. Such produced a common mode error that did not affect the
position. However, the time would was set wrong by one millisecond.
The other grid points fail to compute position properly.
When only codephase is available, and no other measured satellite
timing information can be obtained, then it is not possible to
observe the super millisecond time errors in the process of the
position fix. For weaker signals where the measurement noise is
higher, position accuracy is only affected when the time error
becomes more than about 50-millisecond. Beyond this time
uncertainty, a no-Z fix strategy is needed to handle the effect of
this time error. For stronger signals, the z-count can be observed
and the time error is significantly smaller. An embedded A-GPS
receiver that receives accurate time does not need to deal with
this problem. The eGPS approach has to work harder to produce the
same result as an A-GPS receiver. But, the eGPS receiver is a more
independent and has the advantage that it works without the deep
cellular infrastructure aiding.
In the Fuz3, sub-1500-km (<5-millisecond) zone, the position
error is known to .+-.5-milliseconds or .+-.1500-km. Such is where
the measured data bit phase (BTT) or z-count is used to extend the
non-ambiguity range of the predicted total pseudorange beyond the
range of the codephase measurement.
If there are three or more Type-1 measurements, then total
pseudorange is completely measured and a position can be computed
independent of the position uncertainty and time uncertainty.
In the extended GPS cases below, it is assumed that there are not
at least three Type-1 measurements. The position availability
depends on the combination of the time error and position error
uncertainties.
For the Mixed Type-1 and Type-2 case, if there is at least one
Type-1 measurement, and enough Type-2 measurements so that a total
of at least three is available, then a position can be computed
independent of the starting time uncertainty because the Type-1 SV
will set time to better than 5-milliseconds. The ambiguity function
will always be less than 10-millisecond so accurate N.sub.set is
always provided. The total pseudorange can be estimated for the
Type-2 SV's when ever there is a Type-1 measurement and the
position uncertainty is less than 5-millisecond.
For the Type-2, TU2 case, if there are at least three Type-2
measurements and Tunc=TU2 (Tunc<5-millisecond), then the total
pseudorange can be estimated for ALL the Type-2 measurements for
the same reason that the ambiguity function cannot be greater than
10-millisecond.
For the Type-2, TU3 case, if there are at least three Type-2
measurements and Tunc=TU3
(5-millisecond<=Tunc<10-millisecond), then the total
pseudorange can be estimated for ONLY the i-th Type-2 measurements
if the ambiguity function Tunc+range
uncertainty(i)<10-millisecond, where each of these uncertainties
are positive.
A procedure to determine if the range uncertainty is less than the
margin of (10-Tunc) for the given position uncertainty is used to
form a circle of positions that defines a surface on the Earth with
a radius of Punc with respect to the center of the surface at the
approximate position. If the satellite range difference between the
approximate position and the grid position is less than (10-Tunc),
then the total pseudorange can be estimated for this satellite
without ambiguity for a Type-2 measurement.
Summarizing, the pseudorange can be estimated for the i-th Type-2
measurement if the ambiguity for the i-th satellite:
|Rgrid(i)-Rcenter(i)|<(10-Tunc) for all points around the
approximate position at a radius of Punc.
In the Fuz4, super-1500-km (>5-millisecond) zone, the position
uncertainty is estimated to be larger than .+-.5-milliseconds or
.+-.1500-km. Here again the BTT or the z-count is needed to extend
the non-ambiguity range of the total pseudorange beyond the range
of the codephase measurement.
Whereas there is a case in Fuz3 with Tu2 where the solution always
exists, the ability to compute N.sub.set for the Type-2
measurements in this region always depends on the satellite
geometry.
If there are three or more Type-1 measurements, then total
pseudorange is completely measured, and a position can be computed
independent of the position uncertainty and time uncertainty.
In the extended GPS cases below, it is assumed that there are not
at least three Type-1 measurements. The position availability
depends on the combination of the time error and position error
uncertainties.
For the Mixed Type-1 and Type-2 case, if there is at least one
Type-1 measurement, and enough Type-2 measurements so that a total
of three is available, then a new ambiguity test called the SV time
set ambiguity function is required on each satellite to determine
if N.sub.set can be predicted confidently.
When time is set with the Type-1 measurement, the usability of the
Type-2 satellite depends on whether all points pass the SV time set
ambiguity test within a radius of the position uncertainty Punc.
The ambiguity test on the i-th Type-2 measurement is: Trust
N.sub.set if:
|[Rgrid(i)-Rcenter(i)]-[Rgrid(TS)-Rcenter(TS)]|<10, for all
points inside and around the approximate position at a radius of
Punc.
The biggest protection radius for Punc occurs when the time set SV
and the N.sub.set SV have the largest dot product. Thus, it should
be clear that if there are two Type-1 SV's, then the dot product
should be calculated for both possibilities of the time set SV and
N.sub.set should be predicted with the time set SV that makes the
largest dot product. Said another way, all Type-2 measurements need
not use the same Type-1 to determine N. However, after N is
determined, only one Type-1 SV should be used to adjust the BTT to
GPS time for all measurements. Such ensures that the time error is
common mode on all measurements in the position fix.
For the Type-2, TU2 or TU3 case, if there is no Type-1 measurement
but there are at least three Type-2 measurements, then the method
is the same for both Tunc=TU2 (Tunc<5-millisecond), and TU3
where Tunc<10-millisecond. In this case, the confidence for each
satellite is evaluated with RTC ambiguity function as described in
FUZ3.
In a weaker environment, the extended fixes first process a minimal
set of the Type-1 or Type-2 measurements. In the case of high
geometric-dilution-of-precision (PDOP), the accuracy may not be
suitable for a high accuracy fix. However, even if the error is
high, this fix can serve a valuable purpose of reducing the
position uncertainty to a level where remaining Type-3 measurements
can be processed in a second fix. Only codephase is needed in the
region where the error is <75-km or <150-km, so FUZ1 or FUZ2
methods can be used. This is the same as the self-aiding fix.
Another strategy to extend the range for each satellite is to allow
multiple values of N, since generally N can only be 3, 4. All the
candidate position solutions are formed from all measurement
combinations when some satellites have more than one. If the
solution is over determined, more measurements than unknowns, then
the one with the minimum sums of squares of a-posteriori
measurement residuals (ARR) is isolated as the best solution. (This
well known APR is a chi-squared statistic with M-N degrees of
freedom.) With the 2-step self-aiding approach, other Type-3
measurements can be added to make the solution over determined and
add integrity. The advantage here is that satellite is not skipped
when it fails the Punc surface.
TABLE-US-00001 Table Of Extended Fix Zone Methods Assume if at
least Type-1 SV's are available, then step 1, do full pseudorange
fix. If additional Type-2 or Type-3, then step 2 do a second FUZ1
or FUZ2 fix FixZone FUZ1 FUZ2 FUZ3 FUZ4 Punc (1/4- (0.5-
(<5-millisecond) (>5-millisecond) (-millisecond) millisecond)
millisecond) Punc(-km) <75-km <150-km <1500-km >1500-km
Meas Simple pivot Grid search then Construct Construct Pre-process
method for bias simple pivot method N.sub.set for Type-2 SV's
N.sub.set for Type-2 SV's. method for bias at best grid loc TUI
Time from: Time from: Time from: Time from: Tunc RTC or RTC or RTC
or RTC or <5- Type-1 SV Type-1 SV Type-1 SV Type-1 SV
millisecond X Type-1, X Type-1, X Type-1, X Type-1, Y Type-2, Y
Type-2, Y Type-2, Y Type-2, Z Type-3 Z Type-3 (X + Y) >= 3 (X +
Y) >= 3 (X + Y + Z) >= 3 (X + Y + Z) >= 3 Fix method: No
ambiguity test Use SV time set classic fix needed ambiguity test to
find SV's that pass at radius Punc TU2 Time from: Time from: Time
from: Time from: 5-millisecond RTC RTC RTC RTC <Tunc Y Type-2, Y
Type-2, Y Type-2 Y Type-2 <10- Z Type-3 Z Type-3 Y >= 3 Y
>= 3 millisecond (Y + Z) >= 3 (Y + Z) >= 3 Fix method: Use
RTC ambiguity Use RTC ambiguity test classic fix test to find SV's
that to find SV's that pass pass at radius Punc at radius Punc TU3
Time from: Time from: Time from: 10- RTC RTC RTC millisecond Y
Type-2, Y Type-2, Need 3 Type-1 SV's <Tunc <50- Z Type-3 Z
Type-3 millisecond (Y + Z) >= 3 (Y + Z) >= 3 Fix method
classic fix TU4 Time from: Time from: 50- RTC RTC millisecond Y
Type-2, Y Type-2, <Tunc Z Type-3 Z Type-3 <10 sec (Y + Z)
>= 3 (Y + Z) >= 3 Fix method No-z fix
Conventional cell-based communication modems, such as the ENFORA
GSM modem allow access to the Internet through a cellular network
operator. An SPS receiver can be integrated with an Enfora GSM
modem to allow connection to the SPS server. These modems do not
provide access to "inside-the-infrastructure" information that is
available to an A-GPS subscriber through the cell-based information
source. The "outside-the-infrastructure" client cannot receive the
cell station position as taught in the Krasner '290 patent.
Fortunately, the cellular base station identity and location area
code (station-ID and LAC) are two examples of identifiers that are
in the GSM signal definition that are commercially available data.
The station-ID is intended to be a locally unique code, but may not
be globally unique. The location area code contains more general
information about the country of operation, a region inside the
country, and a code to identify the cellular operator or provider.
While there is no guarantee that the cellular operators will have
unique station-ID's, the combination of the station-ID and location
area code would better provide a unique identity in a given region
that is accessible. However, additional methods might be required
to guarantee uniqueness and avoid ambiguity.
First of all, like a conventional SPS receiver, the eGPS receiver
has a separate low cost oscillator as a frequency source for signal
downconversion and sampling, and a 32-kHz watch-like low power
oscillator to maintain accurate time between positioning sessions.
It can fix any time independent of access to a cellular network for
time and frequency information.
Secondly, it has the ability in hardware and software to demodulate
the SPS data message. The timing information allows the receiver to
measure total pseudorange and thus, determine its position
autonomously without aiding. Furthermore, it can also collect
satellite position information needed in the receiver position
calculation.
In the process of determining the message data, it must also
determine the data bit phase or the so-called bit transition time
(BTT) that indicates the received fractional phase of the data
message bit. For GPS, the 50-bits per second data bit rate yields a
period of 20-milliseconds. A typical total range is
60-85-milliseconds. Such range can also be expressed as 3 or 4
integer data bits, plus 0-19 fractional data bit range (BTT).
Also, BTT can be measured at a much weaker signal level than the
timing message. The timing message requires every bit to be
observed correctly which is difficult with a signal below -145 dBm
as the noise in even a 20-millisecond interval can be occasionally
stronger than the signal. However, because each data bit transition
produces an independent BTT measurement, and there are up to 50 bit
transition per second, these measurements can be combined over a
long period of time. A BTT estimated over 1-15 seconds can allow an
accurate BTT measurement down to a signal level of -150 dBm. Also,
only a high sensitivity receiver like the mobile client 124, 134,
136, can track the frequency at this signal level so that frequency
error is maintained under half of the message data rate.
Combined with a previously computed receiver position, the measured
data bit phase BTT can be used to estimate the total pseudcrange
when the position and timing uncertainty are each less then one
forth of the BTT range, or when the line of sight effect of each is
less than ten-milliseconds. Thus, in degraded conditions where the
timing message cannot be reliably demodulated, the BTT can be used
to predict a pseudorange and allow a positioning capability of at
least .+-.1500-km and generally higher from a previous
position.
If the signals are too weak to provide BTT, then the position can
still be computed within .+-.150-km with a grid search method that
allows the ambiguity effect of the SPS receiver clock time error to
be eliminated from the fractional pseudorange measurements. The
grid search isolates a position that is within .+-.75-km and then a
simple wrap check is sufficient to remove the any wrap of the sub
millisecond clock offset so that the clock offset is common mode in
all measurements.
Thirdly, if signal conditions are degraded, it can also request SPS
satellite data from the SPS data server. The data sent by the
server are in the form of a reduced size model so that the accuracy
is degraded slightly from the true ephemeris to avoid infringement
of complete ephemeris aiding patents.
The net effect is that the eGPS receiver greatly extends the range
of a conventional GPS receiver with connection through the Internet
to a GPS server that is not tied to the cellular infrastructure or
the cellular network's cell-based information source.
In a tracking system application, the system is configured with
three time intervals to maintain high sensitivity but with a low
connection rate to an aiding server. The first interval is called
T.sub.fix and describes the regular period at which a position is
attempted. The T.sub.fix may be selected for maximum sensitivity.
In this case, the T.sub.fix is related to the time it takes for the
position uncertainty to grow to .+-.150-km, since this is the limit
of using the last position as the approximate location in the most
difficult (weakest) signal environment where measuring total
pseudorange is impossible. For an application in Japan, the bullet
train dictates the maximum user velocity of around 300-km/hr. Thus,
it only takes 30 minutes to exceed the 150-km range. Thus a
T.sub.fix of 10 minutes provides three chances that a fix can be
made in the worst signal environments before the 150-km uncertainty
is encountered. It is implied that receiver will only attempt a fix
for a duration T.sub.on, typically a few minutes, to avoid
consuming power when the conditions are too tough for a fix.
The second interval is T.sub.data defines how often the eGPS
receiver makes connect to the position registration server 108 for
satellite position information. Typically a collected ephemeris is
good for four hours before the accuracy deteriorates. Thus, a
typical rate is every two hours. To reduce data traffic, the eGPS
receiver can chose to request only those ephemeris that it has not
been able to collect recently.
The T.sub.report defines how often the position results are
reported to an application or back to the server. If Tfix is set
much lower than Treport, the system can be configured to send all,
or fewer points, e.g., the last point, or average point. A tracking
application may only want to know the position once a day. In
another application an alert is sent when the receiver leaves a
designated area. Thus, it may have a fast fix rate to check the
situation frequently, but only report if the alert criteria is met.
In this case, Treport may also be used to trigger a "still alive"
message needed to know the system is still operating properly.
For a mobile client 124, 134, 136, where the position requests are
under human control, the Tfix may still be active in the background
to keep the position uncertainty from growing unbounded, or it may
also be set to never activate, so that the user has complete
control of the positioning requests. The user will always have
control over whether breadcrumbs will be shared with the server. A
mobile client 124, 134, 136, whose position is computed locally
will have inherently better privacy if its applications don't share
their positions with applications that run on the server. The data
requesting interval can also be adjustable by the user. Periodic
data retrieval will reduce waiting time on position session
activated by the user if the eGPS receiver already has current
satellite data and local breadcrumbs.
If the receiver is in open sky or degraded conditions, the eGPS
receiver will generally be able to fix without any aiding. Aiding
in this environment can speed up the fix time, but it is not needed
to enable the fix. In eGPS however, if the signals are weak and the
SPS cannot receive enough timing information from the satellites it
can track, then its ability to fix is dependent on the position
uncertainty that grows as a function of the time since the last
position calculation. EGPS can extend operation in this area
because it can do a very wide autonomous relative fix around the
last position with the satellite data bit timing information from
as little as three satellites at or above -150 dBm. Without this
requisite additional satellite timing, the eGPS receiver without
breadcrumbs can only operate within 150-km of uncertainty from its
last fix. Even if the position uncertainty grows large, the
frequency search can always be expanded to a point where
acquisition is still possible. The only cost is a reduced
acquisition time. But the real detriment of a high position
uncertainty is the necessity to receive additional timing
information from the SPS satellites besides the codephase to
resolve the total pseudorange. When the uncertainty is too high and
satellite timing is not available, then even though the receiver is
able make the fractional pseudorange measurements, it still cannot
fix because it cannot predict the total pseudorange without
ambiguity.
Because the distance from a tower is never more than 150-km, the
A-GPS receiver has the advantage that it can fix anytime
independent of the signal level as long as the communication
receiver is in contact with the cellular network that has a
cell-based information source However, this requires an extensive
monitoring of the cell base stations and may not provide ubiquitous
operation across networks where the operators have not installed
the cell-based information source.
One way to greatly improve the situation is to have reference
positions where the receiver has already computed a location and to
know when the receiver is again nearby that region so as to permit
a relative fix when an autonomous fix is not possible.
Such a reference location is a breadcrumb, and reception of such
information causes the eGPS receiver to search around this point up
to a 150-km uncertainty and then use its grid search fix algorithm
to fix using only fractional pseudorange information at the closest
grid location. Thus, timing information to form total pseudorange
is not required. Such means positioning is possible even in the
weakest signal conditions when the SPS timing data message cannot
be reliably received.
One method of storing and retrieving breadcrumbs is for the client
to save and name reference positions in a database. Then later when
the client returns to the vicinity of that breadcrumb, it can
improve the ability to fix in that region by indicating to the
receiver that it is near a stored reference point.
Another approach is to associate a name with a unique and specific
city, town, or point of interest in a global sense. Such large
database would be available at the server. The user can search
through a directory of unique places, and if the user indicates
they are in the proximity of this named identifier, and the
reference position for breadcrumb can be retrieved from database,
then the user does this to get a high sensitivity fixes capability
in this vicinity. Continued indication of proximity via a button
push keeps the position uncertainty from growing after the
breadcrumb is received.
A further method embodiment of the present invention uses the cell
base station to identify a breadcrumb. If the user is able to get a
fix and then read this station-ID in a reasonable period around the
fix, it can then associate this station-ID with the previous
position. Thus at a later time, if it can read that ID, then it can
recall this position from its database or the server and the
receiver can then get its highest sensitivity in this region.
For certain types of breadcrumbs, a small uncertainty can
associated with the location. For example being in a small suburb,
or at a small point of interest, or being near a cellular base
station produces a small uncertainty about the distance between the
true location and the center of the point of interest.
Also, in the case of cell sites, the ID may be read some time after
the last SPS receiver fix. In the fast-moving Bullet Train case, a
new cell site encountered five minutes after a previous GPS fix
would have an associated uncertainty of (cellRange+300-km/hr*
5/60)=cell range+5-km.
It can also share its breadcrumbs and identifier with other clients
so that it can also receive the breadcrumbs from other users and
expand its own database.
Embodiments of the present invention can take many forms. For
example, a method of operating the mobile client 124, 134, 136, to
extend its capabilities is firstly to obtain the most timing
information possible from the SPS signals, even in weak signal
conditions. This in order to fix autonomously from with the highest
level of uncertainty in its starting position. Secondly, when it is
able to fix, it updates a database of fresh reference points and
identities. These are thereafter used as approximate starting
locations on subsequent fix sessions started in the vicinity of the
same reference point.
A method for extending the position capability of an SPS receiver
includes computing a fix with at least three Type-1 measurements,
regardless of position uncertainty. Then, computing a position fix
if there is at least one Type-1 SV and enough other Type-2 SV's so
that there are at least 3 SV's with total pseudorange. Same as
saying at least one SV with a z-count and then enough others with
measured data bit phase (BTT). If no Type-1's are available,
computing a position fix if there is at least three Type-2 SV's and
an accurate local real time clock. Same as saying at least three
SV's with measured data bit phase BTT. Then, determining a total
pseudorange using a codephase, a measured fractional data bit (BTT)
and a predicted number of data bits based on an approximate
location. Then, determining a predicted number of data bits where
time is set by either reception of satellite time message or an
accurate local real time clock. And, determining a predicted number
of data bits with a measurement of the fractional data bit phase.
The data bit phase is estimated with a long term histogram to
improve the sensitivity so that the phase can be estimated for a
weaker satellite. An ambiguity function is used to determine the
confidence in an estimated number of data bits.
Multiple candidates are computed for the number of data bits and
then forming multiple full-pseudorange position fixes candidates.
The best candidates are selected for each satellites based on a
test statistic when the full pseudorange fixes are over determined.
The test static is the minimums sums of squares of a posteriori
measurement residuals (APR).
The selection of the best candidates for N for each satellite is
based on adding remaining Type-3 measurements to produce an over
determined fix and then using the APR test statistic. An SV time
set ambiguity function is used to determine the confidence in N
when time is set from the received time message from at least one
satellite (a Type-1 measurement). An RTC ambiguity function is used
to determine the confidence in N when time is set from a local real
time clock. A second step position calculation is used to add
integrity and accuracy by adding Type-3 measurements to the fix
that could not be processed in a first full pseudorange based
position calculation step when the position uncertainty after the
first step is reduced to the range of ambiguity of the
codephase.
When the starting combination of position uncertainty and time
uncertainty exceed the ambiguity of the codephase measurement, a
grid search technique can be to isolate an improved approximate
location where the combination of position uncertainty and time
uncertainty do not exceed the ambiguity of the codephase
measurement.
A pivot wrap method can be used to unwrap the clock bias effect in
the codephase when the combination of position uncertainty and time
uncertainty do not exceed the ambiguity of the codephase
measurement.
A fix interval T.sub.fix is the first of a three interval system
that maintains the position uncertainty below a level so that the
codephase is adequate to measure the position change with no
ambiguity.
A data request interval T.sub.data is the second of a three
interval system at which time a reduced satellite position
ephemeris is requested for any or all satellites from an position
registration server 108 with the Internet via a cellular modem in
order to ensure that the models are never older than their range of
usability (typically 4 hours). This reduced model does not contain
all the parameters of the transmitted ephemeris so that the
accuracy is degraded slightly.
A data report T.sub.report interval is the third of a three
interval system at which the position is reported. A means of
building a local database of breadcrumb reference points using the
autonomous fix capability. After a fix is computed, the SPS
receiver may request a cell station identity from the cellular
modem being used to connect to the position registration server 108
via the Internet. Or the client can manually associate a specific
unique name for the location.
Embodiments of the present invention share the local database with
a master server database so that the master can build a database
that is the union of all clients local database. The client can use
the server to store and retrieve their lifetime specific
breadcrumbs. A configurable number of breadcrumbs can be obtained
from the server so that the client will always have all the
breadcrumbs that could be covered in the period until the next
update.
The server (and or client) can filter algorithms on the database to
handle non-unique breadcrumbs and inactive cell sites. The server
(and or client) can filter algorithms on the database to improve
the position estimate and an estimate of its common range of use.
The server can include additional unique breadcrumbs of unique
points of interests such as cities, tourist locations where there
is a unique name for such a location. Positions for such
breadcrumbs names are generated manually by operators at the
position registration server 108 or by clients who send a position
and a unique breadcrumb name. For example, "SFO" for San Francisco
Airport. Receiving this breadcrumb assures good fixing capability
inside the airport needed when someone is there to take them
home.
At the start of a positioning session, a client can either receive
a breadcrumb from the user database, or may read the station-ID and
request an approximate position from the GPS server. If the
breadcrumb uncertainty is less than the user uncertainty, then the
approximate position is adopted to set the FUZ.
The processing time to acquire at least one SPS satellite can be
reduced using a cell-based communication receiver by, (1) obtaining
a breadcrumb identity, (2) using a current or recent cellular
identity, (3) using a unique breadcrumb name supplied by the
user.
The extended GPS (eGPS) SPS receiver architecture has high
sensitivity like the A-GPS receiver, and can maintain that
sensitivity without the constant aiding required by the A-GPS
receiver. It is able to extend its ability to compute position for
longer periods that would be possible for a conventional receiver
or A-GPS receiver. Also, it can operate much longer in difficult
conditions without aiding that is difficult for the A-GPS
receiver.
This Disclosure has mentioned BLUETOOTH piconets and equipment by
name, but it can be expected that other present and future
technologies would suffice just as well. For example, the CHIRPS
developments by Nanotron (Germany) and the IEEE standard
802.15.4a.
So, although the present invention has been described in terms of
the presently preferred embodiments, it is to be understood that
the disclosure is not to be interpreted as limiting. Various
alterations and modifications will no doubt become apparent to
those skilled in the art after having read the above disclosure.
Accordingly, it is intended that the appended claims be interpreted
as covering all alterations and modifications as fall within the
"true" spirit and scope of the invention.
* * * * *
References